The present invention relates to abrasive articles, particularly grinding wheels, having a mixture of two different abrasive grains. The first abrasive grain comprises seeded or nucleated alpha alumina, preferably iron oxide nucleated alpha alumina, and the second abrasive grain comprises rare earth oxide modified alpha alumina.
Fused alumina abrasive grains or particles have been utilized in abrasive applications for close to one hundred years. Fused alumina abrasive grains are made by heating an alumina source above its melting point, quenching, and then crushing to form the alumina abrasive grains. In the early 1980's a new and substantially improved type of alumina abrasive grains, commonly referred to as "sol gel" or "sol gel-derived" abrasive grains, were commercialized. The sol gel abrasive grains were based upon a sintering process rather than a fusion process. In general, sol gel abrasive grains are made by preparing a dispersion or sol comprising water and alumina monohydrate (boehmite), gelling the dispersion, drying the gelled dispersion, crushing the dried dispersion into particles, calcining the particles, and sintering the calcined particles at a temperature below the melting point of alumina. Frequently, the dispersion also includes one or more oxide modifiers, nucleating agents, and/or precursor thereof. Further details regarding sol gel abrasive grain, including methods for making them, are reported in U.S. Pat. Nos. 4,314,827 (Leitheiser et al.), 4,623,364 (Cottringer et al.), 4,744,802 (Schwabel), 4,881,951 (Wood et al.), 5,011,508 (Wald et al.), 5,090,968 (Pellow), 5,201,916 (Berg et al.), 5,213,591 (Celikkaya et al.), and 5,366,523 (Rowenhorst et al.).
Sol gel abrasive grains are typically incorporated into abrasive articles such as coated abrasive articles, nonwoven abrasive articles, and bonded abrasive articles. Bonded abrasive articles include grinding wheels, stones, hones, and cut-off wheels. The bonds used to hold or bond the abrasive grains within the wheel are typically a resin or organic polymers, but may also be inorganic materials such as ceramics or glasses (i.e., vitreous bonds).
Cut-off wheels are typically small thin wheels used for general cutting operations. The wheels are generally several inches in diameter and a few millimeters thick. They may be operated at speeds from about 1000 to 50,000 RPM, and are used for operations such as cutting metal or glass, for example, to nominal lengths. Cut-off wheels are also know as "industrial cut-off saw blades" and, in some settings such as foundries, as "chop saws".
Examples of commercially available cut-off wheels include those available from Minnesota Mining & Manufacturing Company of St. Paul, Minn.(hereinafter referred to as "3M") under the trade designation "General Purpose Cut-Off Wheel". These 3M cut-off wheels utilize fused aluminum oxide abrasive grain. Other commercially available cut-off wheels include wheels available under the designation "Vulcan" from United Abrasives Inc., of Vulcan, Mich., and wheels from Spedecut of Adelanto, Calif. Both of these examples use essentially 100% brown fused aluminum oxide as the abrasive grain. Cut-off wheels such as these are typically used for cutting of softer workpieces such as mild steel (e.g., 1018 MS) and glass.
In an effort to improve the cutting performance and to reduce the cost of the cut-off wheels, manufacturers also produce wheels which have a blend of two or more types of abrasive grain. In some instances a premium, expensive, abrasive grain is partially replaced with a lower quality, less expensive, abrasive grain. For example, U.S. Pat. No. 3,867,795 (Howard) discloses cut-off wheels and methods of making cut-off wheels. These cut-off wheels may contain a blend of fused aluminum oxide and fused alumina-zirconia abrasive grains. Alumina-zirconia is a mid-priced abrasive grain particularly useful in cutting nickel alloys or stainless steel materials. Wheels available under the trade designation "GREEN CORPS" from 3M utilize a blend of equal amounts of fused alumina abrasive grain and an iron oxide nucleated ceramic abrasive grain which contains approximately 4.5% magnesium oxide. Norton Company, of Worcester, Mass., produces a cut-off wheel containing equal amounts of brown fused aluminum oxide and fused alumina-zirconia abrasive grain. Also available from Norton, under the trade designation "Medallion", are cut-off wheels having equal amounts of fused alumina-zirconia abrasive grain and seeded ceramic aluminum oxide grain (the ceramic aluminum oxide grain being commercially available from Norton as loose abrasive grains under the trade designation "SG").
Ceramic aluminum oxide abrasive grains, which are derived from a sol gel process, are particularly useful in cutting carbon steel articles. By combining the premium abrasive grains with lower quality abrasive grains, the overall cost of the wheel, based on the cost of the abrasive grains used, is reduced. Unfortunately, use of lower quality abrasive grains in the wheel may also reduce the cutting performance of the wheel.
The abrasive industry is continuously searching for ways to improve the performance of cut-off wheels by using existing premium abrasive grains, yet maintain good product performance and still keep the cost at acceptable levels. It would be desired to make a wheel which works well on various types of articles yet still has a reasonable price.